MANILA – China has been reclaiming more land to bolster its military presence in the disputed Spratly Islands in the South China Sea, where its increasing assertion of its territorial claims has brought it into standoffs with its neighbors, Philippine officials said Thursday.

The Philippines protested in April after discovering that Chinese dredgers had expanded the size of the usable land at Johnson Reef in the Spratlys, which could become the basis for a military outpost or an airstrip far from the Chinese mainland.

President Benigno Aquino III said he was disturbed to see surveillance photos of ships capable of reclaiming land near two other Chinese-occupied reefs in the Spratlys, called Cuarteron and Gaven.

“We are again bothered that there seems to be development in other areas within the disputed seas,” Aquino said at a news conference.

When asked whether reclamation of land was underway in the two reefs, Aquino did not give a clear reply, but two military officials said government surveillance had monitored land reclamation activities in Cuarteron and Gaven.

In the Senkaku Islands on Friday, two Chinese coast guard ships entered Japanese territorial waters, the Japan Coast Guard said. It was the 13th intrusion by Chinese government vessels this year and the first since Sunday.

The two ships crossed into Japanese waters near one of the five islands, Uotsuri, around 10 a.m. A Japanese patrol vessel told them to leave, but they ignored the order.

The Chinese ships left the waters around noon, taking a route past Minamikojima, another islet in the chain.

China, the Philippines, Brunei, Malaysia, Taiwan and Vietnam have overlapping territorial claims in the Spratlys, a group of mostly barren islands, reefs and atolls that are believed to be sitting atop deposits of oil and natural gas. They also straddle some of the world’s busiest sea lanes.

Taiwan, Vietnam and the Philippines occupied separate islands in the archipelago decades ago. China later stepped up efforts to take control of uninhabited submerged reefs by reclaiming land and constructing buildings on them that resembled military outposts.

Southeast Asian countries have failed so far to convince China to negotiate a legally binding code of conduct aimed at discouraging actions that could escalate to fighting in the disputed waters.